In Cold War England, Professor Henry Lytten, having renounced a career in espionage, is writing a fantasy novel that dares to imagine a world less fraught than his own. He finds an unlikely confidante in Rosie, an inquisitive young neighbor who, while chasing after Lytten's cat one day, stumbles through a doorway in his cellar and into a stunning and unfamiliar bucolic landscape—remarkably like the fantasy world Lytten is writing about. There she meets a young boy named Jay who is about to embark on a journey that will change both their lives. Elsewhere, in a distopian society where progress is controlled by a corrupt ruling elite, the brilliant scientist Angela Meerson has discovered the potential of a powerful new machine. When the authorities come knocking, she will make an important decision—one that will reverberate through all these different lives and worlds.